Electric multibreak forming cartridge fuse

ABSTRACT

A fuse for elevated circuit voltages has a fuse link provided with a link-severing low fusing point overlay which forms a break on the occurrence of very small overloads the duration of which is in the order of, or exceeds, one hour. The fuse further includes means forming series breaks upon formation of an initial break in the fuse link. These means include a shunt across the overlay portion of the fuse link adapted to generate relatively large amounts of heat at the ends thereof conductively connected to the fuse link and adapted to generate relatively small amounts of heat per unit of shunt length in the center region of the shunt.

United States Patent 1191 1111 3,735,317 Jacobs, Jr. May 22, 1973 [541ELECTRIC MULTIBREAK FORMING 2,416,428 2 1947 Boothe ..337 291 CARTRIDGEFUSE 3,621,433 11/1971 Belcher ..337/244 [75] Inventor: llafilip C.Jacobs, Jr., Newtonville, Primary Examiner Bemard A Gilheany AssistantExaminer-F. E. Bell [73] Assignee: The Chase-Shawmut Company,Att0rneyErwin Salzer Newburyport, Mass. 22 Filed: May 1, 1972 [57]ABSTRACT A fuse for elevated circuit voltages has a fuse link pro- [211Appl' 249707 vided with a link-severing low fusing point overlay whichforms a break on the occurrence of very small [52] US. Cl. ..337/291,337/161, 337/296 overloads the duration of which is in the order of, or[51] Int. Cl. ..H0lh 85/04 exceeds, one hour. The fuse further includesmeans [58] Field of Search ..337/ 161,291,293, forming series breaksupon formation of an initial 337/296 break in the fuse link. These meansinclude a shunt across the overlay portion of the fuse link adapted to 1References Cited generate relatively large amounts of heat at the endsthereof conductively connected to the fuse link and UNITED STATESPATENTS adapted to generate relatively small amounts of heat 876,2731/1908 Hall ..337/29l per unit of shunt length in the center region ofthe 1,818,382 8/1931 Conrad ..337/29l shunt,

2,243,107 5/1941 Lindell ..337/29l 2,341,865 2/ 1944 Hermann ..337/291 4Claims, 5 Drawing Figures 111 111 b b b l l a l O O O O 0 O O O o C IIIa Pmimiw 3', 7 35.317

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ELECTRIC MULTIBREAK FORMING CARTRIDGE FUSE BACKGROUND OF THE INVENTIONThe interruption of very small overload currents in high-voltage fuses,e.g., fuses having a voltage rating of 5-15 kV is, in many instances,effected by means of an arc gap formed by separation of a pair ofrelatively movable or separable contacts. The provision of suchswitching means considerably complicates the design and manufacture offuses.

It is, therefore, one object of the invention, to provide fuses forelevated circuit voltages, capable of interrupting overload currents assmall as the currents which must be carried for one or several hours tocause interruption or clearing of the overloaded circuit by theparticular fuse, without relying on relatively movable or separablecontact to achieve this end.

SUMMARY OF THE INVENTION Electric fuses embodying this invention includea tubular casing of electric insulating material and a pair of metallicterminal elements closing the ends of the casing. A first fusibleelement having a relatively small resistance and forming a plurality ofserially related points of reduced cross-sectional area interconnectsconductively said pair of terminal elements. Said first fusible elementhas means at a predetermined point thereof for causing formation of abreak therein at the occurrence of small protracted overload currents,the fuse further includes a second fusible element having a relativelyhigh resistance shunted across said predetermined point of said firstfusible element. Said second fusible element has ends immediatelyadjacent said first fusible element conductively connected to said firstfusible element. The aforementioned ends of said second fusible elementhave a smaller resistance per unit of length than the center portion ofsaid second fusible element to cause melting of, and consequent arcinitiation by, said second fusible element at points thereof locatedimmediately adjacent said first fusible element.

BRIEF DESCRIPTION OF THE DRAWINGS BRIEF DESCRIPTION OF PREFERREDEMBODIMENT Before describing the preferred embodiment of the invention,a brief summary of the pertinent prior art is given below.

In FIGS. la-lc reference characters have been applied to indicatediagrammatically a pair of metallic terminal elements, e.g., a pair offerrules or a pair of terminal plugs.

According to FIG. 1a terminal elements a are conductively interconnectedby a ribbon fuse link b and by a fusible wire c. The former has arelatively small resistance and may be provided with serially relatedpoints of reduced cross-sectional area. The fusible wire c has arelatively high resistance and its cross-section is uniform along itsentire length. Normally wire c does carry hardly any current. Uponfusion of link b and interruption of the current path formed by it wirec becomes current carrying and melts, thus completing the interruptionof the faulted circuit initiated by fusion of ribbon link b. Thearrangement of FIG. 1 has various applications, but does not lend itselfto effectively interrupt extremely small overload currents whoseduration is, or exceeds, 1 hour.

The structure of FIG. 1b is similar to that of FIG. 1a, except thatribbon fuse link b is provided with an overlay b of a low fusing pointmetal, e.g., tin, capable of severing upon fusion thereof fuse link b bya metallurgical reaction, and except that high resistance wire c' shuntsbut the center portion of fuse link b with its overlay b rather than itsentire length. Upon formation of a break at b' the entire current iscarried by wire c which then fuses relatively rapidly and therebyeffects final interruption or clearing of the faulted circuit. Thestructure of FIG. lb is inapplicable to interruption of very smalloverload currents at relatively high circuit voltages, e.g., 5-15 kVbecause the single point of break formed at b is likely to break downfollowing fusion of wire 0'.

The structure of FIG. 10 includes ribbon fuse link b having a low fusingpoint link-severing overlay b near its center. Reference character c"has been applied to indicate a wire which is normally not conductivelyconnected into the circuit of the fuse. The ends of wire 0 are spacedfrom ribbon fuse link b and form a pair of spark gaps d. Upon fusion ofoverlay b fuse link b is severed at its center resulting in kindling ofan are. When its arc voltage is sufficiently high, spark gaps d breakdown and the resulting arcs formed at spark gaps d sever fuse link b atthe two points b", thus producing multibreaks, e.i., three series breaksare ultimately formed in fuse link b.

To achieve the desired result the potential of resistance wire 0" cannotbe floating. The fuse must be provided with means not shown maintainingwire c" at such a potential that arc gaps d break down followingformation of a break at b and following a predetermined burnback of thearc kindled at b'. A more serious limitation of the structure of FIG. 10resides in the fact that its operation depends critically upon thegaplength of arc gaps d and on the configuration of the surfaces ofparts b and c" at which the voltage gradient between them is largest. Itis difficult to establish in fuses arc gaps d whose breakdown voltage issufficiently uniform. Another limitation of the arrangement of FIG. 1cresides in the fact that a large portion of the heat generated in were0" by F-r losses is lost inasmuch as it is not used to sever fuse link bat points b". Only the heat of the arcs across gaps d is used forforming series breaks b" in fusible element b, but the heat dissipatedfrom wire c" into the pulverulent arc-quenching filler (not shown)surrounding wire c" serves no purpose.

The structure of FIG. 2 is not subject to the aforementioned limitationsof the structure of FIG. 10. As shown in FIG. 2, ribbon fuse link b isprovided at its center region with a low fusing point link-severingoverlay b. The portion of ribbon fuse link b supporting overlay b isshunted by a wire shunt c. The ends of shunt c are conductivelyconnected e.g., -spotwelded or soldered to ribbon fuse link b at thepoints b thereof. The ends of shunt c' conductively connected at b toribbon fuse link b have a relatively high resistance per unit of lengthand generate a relatively large amount of heat when shunt c' becomescurrentcarrying following formation of a break at b. The center portionof shunt c has a relatively small resistance per unit of length andgenerates a relatively small amount of heat when shunt c becomescurrentcarrying following formation of a break at b. As shown in FIG. 2the ends of shunt connected to fusible element b are formed by singlewires, and the center portion of shunt c' is formed by two or more wiresconnected in parallel and has a much smaller resistance than the singlewire ends of shunt c.

The ends of wire shunt 0 formed by a single wire are much shorter thecenter of wire shunt c formed by a plurality of wires. As a result, thegeneration of heat is concentrated to relatively small regionsimmediately adjacent ribbon fuse link b.

Upon formation of a break at b shunt 0 becomes current-carrying. Thisresults in fusion of its high resistance ends conductively connected toribbon fuse link b at the two points b". The resulting arcs sever ribbonfuse link b at points b. Points 12" where shunt c' is conductivelyconnected to ribbon fuse link b are preferably points where thecross-sectional area of the latter is reduced.

In certain instances it is possible and desirable to provide ribbon fuselink b at points b with low fusing point overlays capable of severingribbon fuse link b by a metallurgical reaction when these overlays reachthe fusing point of the overlay forming metal, e.g.,- tin. The provisionof such overlays at points b' is possible and meaningfull in instanceswhere the temperature distribution along fusible element b is generallymore or less of a flat top nature or, in other words, where thetemperature at points b is substantially equal to, or but slightly lessthan, the temperature at point b. Then the generation of heat at theends of shunt 0" may raise the temperature at points b of ribbon fuselink b sufficiently fast to sever fuse link b at points b' by ametallurgical reaction between the overlay metal at points b and thebase metal at points b only shortly after such a metallurgical reactionhas taken place at b.

Referring now to FIG. 3, the structure shown therein is substantiallythe same as that shown and described in U.S. Pat. 3,621,433 to RichardA. Belcher, Nov. 16, 1971 except for the addition of the low overloadcurrent interrupting multibreak-forming means shown in FIG. 2 anddescribed in connection with that figure.

Numeral 1 has been applied in FIG. 3 to indicate a tubular casing ofelectric insulating material closed on both ends thereof by terminalplugs 2 press-fitted into casing 1. Terminal plugs 2 are firmly held inposition by steel pins 3 projecting through casing 1 into terminal plugs2. The axially inner sides of terminal plugs 2 are provided with grooves2a which receive the axially outer ends of four ceramic plates 4 of aheat-shock resistant material. Plates 4 form a spider, or mandrel, forsupporting a ribbon fuse link b. The latter is helically wound aroundthe radially outer edges of the spiderforming plates 4 and conductivelyinterconnects terminal plugs 2. The plate-mandrel structure of FIG. 3 isdisclosed more in detail in U.S. Pat. No. 3,599,138 to Frederick J.Kozacka, Aug. 10, 1971 for HIGH- VOLTAGE FUSE. The ribbon fuse link b ofsilver is provided in its center region with a low fusing pointlink-severing overlay b. Shunt c extends across a section of ribbon fuselink b to both sides of overlay b. The ends of shunt 0" having arelatively high resistance per unit of length and a relatively smallcrosssectional area are aflixed to points of ribbon fuse link b wherethe cross-sectional area of the latter is reduced by means of one of aplurality of circular perforations provided therein. Casing 1 is filledwith a pulverulent arc-quenching filler 5, preferably quartz sand. Bothterminal plugs 2 are provided in the center thereof with an internallyscrew-threaded bore of which each receives a plug screw 7. One of plugscrews 7 is provided with a recess receiving a spring-biased blown fuseindicator and the other of plug screws 7 receives a clamping means foranchoring the end of a restraining wire for the blown fuse indicatorremotefrom the blown fuse indicator. The details of these features aredescribed in the above referred-to U.S. Pat. No. 3,621,433 to Richard A.Belcher. The hexagonal heads of plug screws 7 clamp the axially innerend surfaces of a pair of ferrules 8 against the axially outer endsurfaces of terminal plugs 2.

On occurrence of overload currents the structure of FIG. 3 operates inthe fashion described above in connection with FIG. 2 first forming onebreak at b and shortly thereafter forming two additional series breaksat b. Major fault currents result in the formation of series breaks, oneat each point of reduced crosssectional area of fuse link b.

I claim as my invention:

1. An electric fuse adapted to form multibreaks at the occurrence ofsmall overload currents including a. a tubular casing of electricinsulating material;

b. a pair of metallic terminal elements closing the ends of said casing;

c. a first fusible element having a relatively small resistance andforming a plurality of serially related points of reducedcross-sectional area conductively interconnecting said pair of terminalelements, said first fusible element having means at a predeterminedpoint thereof for causing formation of a break therein at the occurrenceof small protracted overload currents;

e. a second fusible element having a relatively high resistance shuntedacross said predetermined point of said first fusible element and havingends immediately adjacent said first fusible element conductivelyconnected to said first fusible element, said ends of said secondfusible element having a smaller resistance per unit of length than thecenter portion of said second fusible element to cause melting of, andconsequent arc initiation by, said second fusible element at pointsthereof located immediately adjacent said first fusible element.

2. An electric fuse as specified in claim 1 wherein said first fusibleelement is formed by a ribbon of silver supporting at said predeterminedpoint a metallic overlay having a relatively low fusing point andcapable of severing upon fusion thereof the current path through saidribbon of silver, and wherein said second fusible element is formed bywire means having a smaller electric conductivity than silver.

3. An electric fuse as specified in claim 2 wherein the ends of saidsecond fusible element immediately adjacent to said first fusibleelement are formed by single wire means and wherein the center portionof said second fusible element is formed by a plurality of wire meansconnected in parallel.

4. An electric fuse as specified in claim 1 wherein the length of saidends of said second fusible element is relatively short in comparison tothe length of said center portion thereof in order to concentrate heatgeneration by said second fusible element to relatively small portionsthereof immediately adjacent said first fusible element.

1. An electric fuse adapted to form multibreaks at the occurrence ofsmall overload currents including a. a tubular casing of electricinsulating material; b. a pair of metallic terminal elements closing theends of said casing; c. a first fusible element having a relativelysmall resistance and forming a plurality of serially related points ofreduced cross-sectional area conductively interconnecting said pair ofterminal elements, said first fusible element having means at apredetermined point thereof for causing formation of a break therein atthe occurrence of small protracted overload currents; e. a secondfusible element having a relatively high resistance shunted across saidpredetermined point of said first fusible element and having endsimmediately adjacent said first fusible element conductively connectedto said first fusible element, said ends of said second fusible elementhaving a smaller resistance per unit of length than the center portionof said second fusible element to cause melting of, and consequent arcinitiation by, said second fusible element at points thereof locatedimmediately adjacent said first fusible element.
 2. An electric fuse asspecified in claim 1 wherein said first fusible element is formed by aribbon of silver supporting at said predetermined point a metallicoverlay having a relatively low fusing point and capable of severingupon fusion thereof the current path through said ribbon of silver, andwherein said second fusible element is formed by wire means having asmaller electric conductivity than silver.
 3. An electric fuse asspecified in claim 2 wherein the ends of said second fusible elementimmediately adjacent to said first fusible element are formed by singlewire means and wherein the center portion of said second fusible elementis formed by a plurality of wire means connected in parallel.
 4. Anelectric fuse as specified in claim 1 wherein the length of said ends ofsaid second fusible element is relatively short in comparison to thelength of said center portion thEreof in order to concentrate heatgeneration by said second fusible element to relatively small portionsthereof immediately adjacent said first fusible element.